The Global System for Mobile Communications (GSM: originally from Groupe Special Mobile) is currently the most popular standard for mobile phones in the world and is referred to as a 2G (second generation) system. Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) mobile phone technologies. Currently, the most common form uses W-CDMA (Wideband Code Division Multiple Access) as the underlying air interface. W-CDMA is the higher speed transmission protocol designed as a replacement for the aging 2G GSM networks deployed worldwide. More technically, W-CDMA is a wideband spread-spectrum mobile air interface that utilizes the direct sequence Code Division Multiple Access signaling method (or CDMA) to achieve higher speeds and support more users compared to the older TDMA (Time Division Multiple Access) signaling method of GSM networks.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of the popular Orthogonal Frequency-Division Multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of sub-carriers to individual users. This allows simultaneous low data rate transmission from several users. Based on feedback information about the channel conditions, adaptive user-to-sub-carrier assignment can be achieved. If the assignment is done sufficiently fast, this further improves the OFDM robustness to fast fading and narrow-band co-channel interference, and makes it possible to achieve even better system spectral efficiency. Different number of sub-carriers can be assigned to different users, in view to support differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user. OFDMA is used in the mobility mode of IEEE 802.16 WirelessMAN Air Interface standard, commonly referred to as WiMAX. OFDMA is currently a working assumption in 3GPP Long Term Evolution (LTE) downlink. Also, OFDMA is the candidate access method for the IEEE 802.22 “Wireless Regional Area Networks”.
NodeB is a term used in UMTS to denote the BTS (base transceiver station). In contrast with GSM base stations, NodeB uses WCDMA or OFDMA as air transport technology, depending on the type of network. As in all cellular systems, such as UMTS and GSM, NodeB contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the mobiles, which move freely around it. In this type of cellular networks the mobiles cannot communicate directly with each other but have to communicate with the BTSs
Traditionally, the NodeBs have minimum functionality, and are controlled by an RNC (Radio Network Controller). However, this is changing with the emergence of High Speed Downlink Packet Access (HSDPA), where some logic (e.g. retransmission) is handled on the NodeB for lower response times and in 3GPP LTE (a.k.a. E-UTRA) almost all the RNC functionalities have moved to the NodeB.
The utilization of cellular technologies allows cells belonging to the same or different NodeBs and even controlled by different RNC to overlap and still use the same frequency. The effect is utilized in soft handovers.
Since WCDMA and OFDMA often operates at higher frequencies than GSM, the cell range is considerably smaller compared to GSM cells, and, unlike in GSM, the cells' size is not constant (a phenomenon known as “cell breathing”). This requires a larger number of NodeBs and careful planning in 3G (UMTS) networks. Power requirements on NodeBs and UE (user equipment) are much lower.
A NodeB can serve several cells, also called sectors, depending on the configuration and type of antenna. Common configuration include omni cell (360°), 3 sectors (3×120°) or 6 sectors (3 sectors 120° wide overlapping with 3 sectors of different frequency).
High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing UMTS protocols. Two standards HSDPA and HSUPA have been established. High Speed Uplink Packet Access (HSUPA) is a packet-based data service of Universal Mobile Telecommunication Services (UMTS) with typical data transmission capacity of a few megabits per second, thus enabling the use of symmetric high-speed data services, such as video conferencing, between user equipment and a network infrastructure.
An uplink data transfer mechanism in the HSUPA is provided by physical HSUPA channels, such as an Enhanced Dedicated Physical Data Channel (E-DPDCH), implemented on top of the uplink physical data channels such as a Dedicated Physical Control Channel (DPCCH) and a Dedicated Physical Data Channel (DPDCH), thus sharing radio resources, such as power resources, with the uplink physical data channels. The sharing of the radio resources results in inflexibility in radio resource allocation to the physical HSUPA channels and the physical data channels.
The signals from different users within the same cell may interfere with one another. This type of interference is known as the intra-cell interference. In addition, the base station also receives the interference from the users transmitting in neighboring cells. This is known as the inter-cell interference
When an orthogonal multiple access scheme such as Single-Carrier Frequency Division Multiple Access (SC-FDMA)—which includes interleaved and localized Frequency Division Multiple Access (FDMA) or Orthogonal Frequency Division Multiple Access (OFDMA)—is used; intra-cell multi-user interference is not present. This is the case for the next generation UMTS enhanced-UTRA (E-UTRA) system—which employs SC-FDMA—as well as IEEE 802.16e also known as Worldwide Interoperability for Microwave Access (WiMAX)—which employs OFDMA. In this case, the fluctuation in the total interference only comes from inter-cell interference and thermal noise which tends to be slower. While fast power control can be utilized, it can be argued that its advantage is minimal.
In the uplink (UL) of OFDMA frequency division multiple access (both classic OFDMA and SC-FDMA) communication systems, it is beneficial to provide orthogonal reference signals (RS), also known as pilot signals, to enable accurate channel estimation and channel quality indicator (CQI) estimation enabling UL channel dependent scheduling, and to enable possible additional features which require channel sounding.
Channel dependent scheduling is widely known to improve throughput and spectral efficiency in a network by having the Node B, also referred to as base station, assign an appropriate modulation and coding scheme for communications from and to a user equipment (UE), also referred to as mobile, depending on channel conditions such as the received signal-to-interference and noise ratio (SINR). In addition to channel dependent time domain scheduling, channel dependent frequency domain scheduling has been shown to provide substantial gains over purely distributed or randomly localized (frequency hopped) scheduling in OFDMA-based systems. To enable channel dependent scheduling, a corresponding CQI measurement should be provided over the bandwidth of interest. This CQI measurement may also be used for link adaptation, interference co-ordination, handover, etc.
One method for forming reference signals is described in US patent application 20070171995, filed Jul. 26, 2007 and entitled “Method and Apparatus for Increasing the Number of Orthogonal Signals Using Block Spreading” and is incorporated by reference herein. The generation of reference signals (RS) sequences can be based on the constant amplitude zero cyclic auto-correlation (CAZAC) sequences, and the use of block spreading for multiplexing RS from multiple UE transmitters is described therein.